I made a Baltic porter and added 8 oz of Grandma's Molasses (by weight, not fluid oz) to the end of the boil. Having tasted a few times when testing gravity and during trans to secondary, I think I want to prime with molasses to get some more from it. I have the formula from Mr. Wizard at BYO (http://byo.com/stories/article/indices/21-carbonation/1726-maple-carbonation-a-pitching-rates-mr-wizard) (I do need to insert the brix for the molasses which I got from the manufacturer) but I don't know if the answer should be by weight or by fluid oz. Any ideas or expieriences?

I had the same issue with a smoked maple barleywine. Added maple syrup at the end of the boil, but at racking to the secondary, the maple was barely detectable. I primed with maple syrup at my usual corn sugar rate plus 25%. The carbonation level is ideal, although the flavor contribution was barely noticeable.